YouTube is deleting thousands of sexually explicit videos after it was hit by an organised attack yesterday in a prank known as "Porn Day".

The video-sharing website, owned by Google, has removed most of the porn clips but some content could be available for days as YouTube deletes the offending material. The pranksters hid the porn amid innocent footage of celebrities such as Hannah Montana and Jonas Brothers.

Members of the 4Chan message board, which focuses mostly on Japanese anime and manga, have claimed responsibility for uploading the porn, apparently in response to YouTube's stance on copyright music videos.

One member told the BBC: "I did it because YouTube keeps deleting music. It was part of a 4Chan raid."

The porn violates YouTube's terms of use, which ban nudity of any form. The policy has proved controversial in the past when medical videos have been removed, including clips of doctors demonstrating how to conduct self-examination for breast or testicular cancer.

A YouTube spokesman said the clips were detected and removed via the normal process, which allows users to flag questionable material to be reviewed by the website's staff. Although YouTube has review teams around the world, he said most of yesterday's work removing the porn content was done in the US because of timezone issues.

"YouTube is a community site used by millions of people in very positive ways. Sadly, as with any form of communication, there is a tiny minority of people who try to break the rules," the spokesman added.

"We were aware of yesterday's issue and removed the videos as they were brought to our attention through our flagging system, as we would any videos that violate our community guidelines. In addition, any account we discover that has been set up specifically to attack YouTube was also disabled."

The spokesman could not reveal how many videos have been removed following yesterday's Porn Day prank but it is reportedly in the thousands. It is understood the videos originated from all over the world but were uploaded on the same day in a co-ordinated effort.

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